A cellulose acylate film is used in various photographic or optical elements because it is tough and has enough flame retardant properties. In fact, the cellulose acylate film is a representative photographic support. Further, having an optical isotropy, the film is also used in a liquid crystal display device, which has recently extended its market. In the device, the cellulose acylate film often serves as a protective film of a polarizing plate or a color filter.
A cellulose acylate film is generally produced according to a solvent casting method or a melt casting method. The solvent casting method comprises the steps of casting a solution of cellulose acylate in a solvent (that is called “dope”) on a support, and evaporating the solvent to form a film. On the other hand, the melt casting method comprises the steps of heating cellulose acylate to melt, casting the melt on a support, and cooling to form a film. Practically, the solvent casting method is generally adopted because it can form a highly flat film, as compared with the melt casting method.
The solvent casting method is described in various documents. In some recent proposed solvent casting methods, it is aimed to shorten the time between the steps of casting the dope on the support and peeling the formed film off. If the time is shortened, the productivity of the film formation is improved. For examples, Japanese Patent Publication No. 5(1993)-17844 discloses a process of casting a concentrated dope on a cooling drum to shorten the time between the casting step and the peeling step.
The solvent used in the solvent casting method must have functions of not only dissolving cellulose acylate but also forming an excellent film. In more detail, the viscosity and the polymer concentration of the solution (dope) should be appropriately adjusted to form a flat film having a uniform thickness. The dope also should have enough stability to have a long shelf life. Further, the dope should be easily set to gel. Furthermore, the formed film should be easily peeled from the support. In order to satisfy these requirements, the most appropriate solvent must be selected. Moreover, the solvent should be so easily evaporated that it scarcely remains in the film.
Various organic solvents have been proposed as the solvents of cellulose acylate. However, only methylene chloride satisfies all of the above-mentioned requirements. Accordingly, solvents other than methylene chloride have not been practically used.
However, the use of chlorinated solvents such as methylene chloride has been recently restricted severely to protect the global environmental conditions. Further, having a low boiling point (41° C.), methylene chloride vaporizes in the film forming process so easily that it may cause problems in the working environment. Accordingly, the process is conducted under closed conditions. However, there is a technical limitation on sealing methylene chloride in a closed system. Therefore, it is an urgent necessity to search for a new solvent for the cellulose acylate solution in place of methylene chloride.
By the way, acetone is a widely used organic solvent. It has an appropriate boiling point (56° C.), and evaporating acetone does not need a large thermal energy. Further, acetone causes few problems on the human body and the global environmental conditions, as compared with the chlorinated organic solvents. However, cellulose acylate has a poor solubility in acetone. Cellulose acylate having a degree of substitution of not more than 2.70 (acetic acid content: 58.8%) is slightly soluble in acetone but the solubility decreases in accordance with increase of the substitution degree. In fact, cellulose acylate having a degree of substitution of more than 2.70 is almost insoluble in acetone. Cellulose acylate having the substitution degree of not less than 2.80 (acetic acid content: 60.1%) is not soluble in acetone, and is merely swelled in acetone.
J. M. G. Cowie et al. report in Makromol., Chem., 143 (1971) 105, that cellulose acylate having a substitution degree in the range of 2.80 to 2.90 is dissolved in acetone by a specific process. The process comprises the steps of cooling the cellulose acylate (in the reported process the acyl group is restricted to acetyl group) in acetone at a temperature of −80 to −70° C., and warming it to obtain 0.5 to 5 wt. % solution of cellulose acylate in acetone. The method, in which the mixture of cellulose acylate is cooled in an organic solvent to obtain a solution, is hereinafter referred to as “cooling dissolution method”.
The solution of cellulose acylate in acetone is also reported by K. Kamide et al., Textile Machinery Society, Vol. 34, pp. 57 (1981). The report (written in Japanese) is entitled “Dry spinning process using acetone solution of triacetyl cellulose”. In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments described in the report examine the mechanical strength, the dyeing property and the sectioned shape of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solution of cellulose acetate is used to form a fiber.
A film of cellulose acylate is generally produced by the solvent casting method, which comprises the steps of casting the above-described solution of cellulose acylate (dope) on a support, evaporating the solvent to form a film, and peeling the film from the support. However, the inventors found that, if the dope is prepared from a non-chlorinated solvent, there is a disadvantage in the production.
A non-chlorinated solvent which can dissolve cellulose acylate has a higher boiling point than methylene chloride, which is a chlorinated solvent generally used as a solvent for cellulose acylate. Accordingly, the non-chlorinated solvent evaporates from the cast dope (film) on the support so slowly that it takes relatively long time to dry the film enough to peel off, as compared with methylene chloride. If the film formed from the dope of non-chlorinated solvent is peeled after the same drying time as a film from the methylene chloride dope, peeling stress is so loaded and the solvent so remains that the film is deformed to obtain an uneven surface.